Human optimized Bacillus anthracis protective antigen

ABSTRACT

The invention relates to a humanized nucleic acid construct from  Bacillus anthracis  protective antigen (PA) gene and method of modifying the gene. The humanized gene, and method of producing it, improves the structural fidelity of expressed protein product, when produced in mammalian host cells, to native, bacterially produced protein. The construct is useful in nucleic acid based vaccine formulations against  B. anthracis.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/260,656, filed 12 Nov. 2009, which is incorporated herein byreference.

BACKGROUND OF INVENTION

1. Field of Invention

The inventive subject matter relates to a codon optimized nucleic acidsequence of Bacillus anthracis protective antigen (PA). The method ofcodon optimization of the gene is aimed at improving expression inmammals, including humans, as well as enhancing immunogenicity againstendogenously produced PA protein. The inventive construct, incorporatedinto DNA expression systems, can be useful as a component of immunogeniccompositions against B. anthracis, such as vaccines.

2. Background Art

B. anthracis, the etiological agent of anthrax, is a spore-forming, grampositive bacterium. Infection can occur through a variety of routesincluding cutaneous and gastrointestinal, however, inhalational anthraxis the most widely recognized and feared (Baillie, J. Appl Microbiol.,91: 609-613 (2001)). Following inhalation, the majority of theaerosolized spores are immediately phagocytized by alveolar macrophagesand transported through the lymphatic channels to hilar andtracheobronchial lymph nodes. This rapidly leads to the multiplicationand systemic circulation of vegetative bacilli. It is believed that enroute to these regional lymph nodes the spores begin to germinate andmultiply within the macrophage.

Advanced stages of infection are predicated on B. anthracis'anti-phagocytic capsule and the secretion of a tripartite exotoxinconsisting of a cell binding component, Protective Antigen (PA), whichbinds to two enzymatically active subunits: Lethal Factor (LF) or EdemaFactor (EF) to form lethal toxin (LeTx) and edema toxin (EdTx),respectively. The currently available licensed human vaccine for B.anthracis (BioThrax) is a filtered extract from B. anthracis absorbed toalum and is primarily composed of PA.

SUMMARY OF THE INVENTION

An object of the invention is a humanized, i.e., codon optimized, DNAconstruct of Bacillus anthracis protective antigen. The modificationsenable efficient translation of PA in mammals, including humans.

Another object of the invention is a method of codon optimizationutilizing rare host codons in place of rare bacterial codons, ratherthan those most highly utilized by the host. This enables ribosomalstalling at appropriate places along the gene to ensure intra-molecularassociations occur within the nascent protein similar to that whichwould occur naturally. Correct folding of PA would result in a moreefficacious immune response against naturally occurring B. anthracisexpressed PA.

A further object of the invention is a method of human optimizationwhereby codon optimization does not consist of replacing all bacterialcodons throughout the length of the gene with the most highly orfrequently used codons in the host cell. Instead, the inventive methodutilizes consideration of a number of factors in order to affordincreased expression efficiency in a mammalian (e.g., human) host cell,as well as an yielding an expressed protein similar in structure to thenative, B. anthracis, PA protein.

The first factor considered is protein expression efficiency. Byincorporating codons that are highly utilized in the mammalian host cellfor the first 50 codons of the bacterial sequence, the mammalianribosome will effectively engage the mRNA, decreasing the likelihood ofearly termination of the ribosome is minimized. Another factor is tomaximize the opportunity for correct protein folding. This is affordedby first searching for regions in the native PA sequence where rarecodons are utilized in the bacterial gene. Regions were rare codons areheavily utilized may result, in normal, native PA expression,specifically proper folding of the expressed protein. In the modifiedsequence, these use of rare codons is maintained by substituting rarecodons from the human bias table. This would permit the ribosome tostall, where it normally would when expressing native PA in thebacteria, and permit normal folding to occur. Another factor is toensure against unwarranted deletions of mRNA. Therefore, the bacterialsequence is analyzed to search for ribosomal splice sites to ensure thatpost-transcriptional machinery of mammalian cells did not deletesections of the mRNA. Finally, an analysis of the bacterial sequence isundertaken to identify any regions of complementarity. These regions areimportant since they could potentially result in the single stranded RNAfolding back on itself resulting in unwanted host cell operations, suchas ribosomal stalling or premature termination of translation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(A-C). Alignment of the human optimized sequence and the parentsequence. Humanized protective antigen (HoPA) (upper sequence) is thehuman optimized gene and PA (lower sequence) is the parent wild-typegene. Asterisks denote nucleotides that align. FIG. 1A-C shows alignmentof by 1-780; 781-1560; and 1561-2208, respectively.

FIG. 2. Evaluation of DNA vaccines. During this 56 day study A/J mice(n=8 per group) injected IM with pDNAVACCultura2™-HoPA encoding thehuman optimized PA gene with the tissue plasminogen activator (TPA)signal sequence elicited a robust anti-PA IgG response during the 14days following the second boost. This response gradually contracted overthe final 14 days of the study. FIG. 3. Anti-PA IgG titers in responseto homologous prime-boost-boost with pDNAVACCultra2™-HoPA. Eight groupsof mice (n=10) were immunized IM with pDNAVACCultra2™-HoPA on threeseparate occasions 28 days apart. Control groups were injected withpDNAVACCultra2 without HoPA, the lipid adjuvant dioleoylphosphatidylethanolamine-dimethyl dioactadecylammoniium bromide(DDAB-DOPE) only, and 10 μg of rPA injected with Alum adjuvant. Titerswere low in comparison to the control rPA treated group. FIG. 4. Anti-PAIgG titers in response to homologous prime-boost withpDNAVACCultra2™-HoPA. Eight groups of mice (n=10) were immunized IM withpDNAVACCultra2™-HoPA on two separate occasions 28 days apart. Controlgroups were injected with pDNAVACCultra2 without HoPA, the lipidadjuvant (DDAB-DOPE) only, and 10 μg of rPA injected with Alum.

FIG. 5. Efficacy of a homologous prime-boost-boost and a prime-boostwith pDNAVACCultra2™-HoPA. Eight groups of mice (n=10) were immunized IMwith pDNAVACCultra2™-HoPA two separate occasions 28 days apart. Fourteendays after the last immunization all mice were challenged with LD₅₀s ofB. anthracis Sterne strain spores. Survival was significantly improvedat 90% with three 100 μg doses of pDNAVACCultra2-HoPA, designated in thefigure legend as 7162-HoPA, relative to the lower less frequent doses.Recombinant PA protected 100% of the mice.

FIG. 6. Anti-PA IgG titers and survival in response to homologousprime-boost-boost with humanized protective antigen.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following terms are defined:

An immunogenic composition is a composition, containing one or moreantigens, including proteins or peptides or nucleic acid expressionsystems that express immunogenic proteins or peptides in vivo for theinduction of a humoral or cell mediated immune response; a vaccine is animmunogenic composition used to induce protective immunity; a DNAexpression system is a molecular system containing plasmid or closedloop DNA containing elements for expressing an inserted DNA sequence aspolypeptide; a viral expression system is any viral based system,including viral like particles or viral replicons, containing elementsfor expressing an inserted DNA sequence as a polypeptide.

Immunization of susceptible individuals through the process ofvaccination has long been the most desirable approach to diseaseprevention. This is particularly important in anthrax sincemanifestation of the disease results in high mortality. Therefore, it ispreferred to prophylactically protect against infection rather thanattempt to administer antibiotic post-infection.

The current licensed vaccine in the U.S. for anthrax is a cellularfiltrate of B. anthracis that mostly contains PA (Baillie, L., J ApplMicrobiol, 91: 609-613 (2001)). Unfortunately, due to the nature of thevaccine, batch to batch variability occurs, resulting in inconsistencyin efficacy.

In order to alleviate these problems, recombinant technology has beenemployed. Nucleic acid based, or DNA vaccines represent a relativelyrecent and attractive vaccination modality. This interest has beenstemmed by their inexpensive and easy production, high stability, andflexibility regarding cloning and delivery method. The basic structureof a DNA vaccine is a plasmid or closed loop of DNA (the plasmid“backbone”) that contains a selectable marker, a mammalian promoter suchas the CMV promoter for tissue specific expression, and the geneencoding the antigen of interest, in this case protective antigen (PA).

In principal, the DNA is delivered to either immunologically relevantcells of the skin or to cells that have highly active transcription andtranslation machinery such as muscle cells where the PA gene isexpressed via the CMV promoter, released and displayed from the cell,and hopefully picked up by a scavenging dendritic cell or macrophage.

Unfortunately, DNA vaccines in primates and humans often do not elicithumoral or antibody based responses (Calarota, et al., Lancet, 351:1320-1325; Coban, et al., Infect Immun., 72: 584-588 (2004); Epstein, etal., Hum Gene Ther, 13: 1551-1560 (2002); Klinman, et al., Curr TopMicrobiol Immunol, 247: 131-142 (2000); Wang, et al., Science, 282:476-480 (1998)) which are critical to surviving an anthrax infection.Several approaches have been developed to enhance the immunogenicity ofDNA vaccines including the use of adjuvants, altering the deliverysystem, modifying the plasmid backbone by including CpG motifs, oraltering the codon bias (Leitner, et al., Vaccine, 18: 765-777 (1999);O'Hagan, et al., Nat Rev Drug Discov, 2: 727-735 (2003)).

The current invention relates to a DNA construct, useful in vaccineformulations, utilizing a novel human codon-optimized PA gene sequence.The current invention, unlike previously described methods of DNAoptimization, not only permits efficient expression of recombinantprotein, but also enables expression of protein tertiary structure, withfidelity to the native bacterially expressed protein. PA expression fromB. anthracis is optimized through consideration of a number of factorsthat enable efficient expression in a mammalian host, e.g., human. Thefactors considered in the process also improve the likelihood of greatertertiary structural similarity between the expressed recombinant proteinand native, bacterially expressed PA. The result is a greater likelihoodof a more efficacious induction of adaptive immunity.

Method for Humanizing DNA Sequence

DNA vaccines rely heavily on the natural processes of transcription andtranslation by the eukaryotic host cell. Bacterial gene structures arevery different from the human host and require specializedtranscriptional and translational apparatuses. These differences includea lack of introns (noncoding regions that eukaryotes splice out ofmessage RNA), the presence of operons (multiple genes in one message),and a variety of secondary structures within the mRNA that are foreignin eukaryotes (Strugnell, et al., Immunol Cell Bio, 75: 364-369 (1997)).Additionally, bacterial proteins are not glycosylated by the bacterialsystem but contain amino acid motifs which are efficiently andinappropriately glycosylated by eukaryotic cells. Bacterial mRNA alsolacks appropriate structures and sequences to insure an effectivehalf-life in eukaryotic cells.

A important additional difference between eukaryotic and prokaryotictranscriptional/translational systems is the significant differences incodon usage and the arrangement of nucleotides in bacterial mRNA thatgive rise to codons that are rare in eukaryotic mRNA (Manoj, et al.,Crit Rev Clin Lab Sci, 41: 1-39 (2004)). These differences may beexplained by the composition of the tRNA pool that is available to thehost for translation or the guanine/cytosine (GC) and adenine/thymidine(AT) percentages of the bacterial gene and their similarity to theeukaryotic host (Saler, Nat Rev Drug Discov, 2: 727-735 2003)).Coincident with these differences is the operation of the ribosome andthe complex combinations of RNAs and proteins that comprise thetranslational machinery.

During translation, the ribosome attaches to the mRNA by a specificrecognition operation. As the ribosome proceeds down the mRNA itspecific codons are recognized leading to a defined assembly of aminoacids to ultimately build the nascent protein. Ribosomes have been shownto complete this protein synthesis in a complex manner moving down themRNA at a varying rate of progression resulting in a multitude ofdifferent structural results. If a ribosome slows in its progression, itdisconnects from the RNA resulting in premature termination oftranslation.

Variations in the rate of ribosomal processivity can result in thecreation of important structural features. For example, pausing isthought to allow proteins to create protein folds, allowing complexintra-molecular associations to occur. These associations can give riseto proper protein function but also create important immunogenic motifs,that are not present from the linear sequence.

However, when bacterial sequences are expressed from eukaryotic hostsystems, variations, away from that seen in the bacteria, can result insignificant differences in ribosomal procession, glycosylation and evenpremature termination of translation. The result, therefore, indeveloping immunogenic compositions, are proteins that may not mimicnative protein immune induction.

In developing a more antigenically efficient PA protein, geneticincompatibilities between bacterial and eukaryotic genomes weremitigated by modifying the bacterial sequence in order to conform tooptimal codon usage in eukaryotic hosts.

There are many approaches that can be taken in the effort to producebacterial gene sequences that are translated in human cells moreefficiently. The most common is to synthesize the new gene sequenceusing only the most highly used codon in the host organism. However,this method does not take into account differences between prokaryoticand eukaryotic transcription and translational machinery or theguanine/cytosine (GC) and adenine/thymidine (AT) content of thebacterial gene.

The approach utilized in the current invention is to modify thebacterial gene in order to permit expression resulting in a greaterlikelihood of maintaining fidelity to the bacterially expressed nativeprotein. This is termed here as “human optimization.” The aim of thisapproach is to produce a recombinant protein with a greater likelihoodof inducing a more efficacious adaptive immune response.

In the inventive method to modify proteins for efficient expression ofantigens in a eukaryotic host a number of factors are taken intoaccount. These are summarized as:

-   -   a. efficiency of translation;    -   b. fidelity of protein folding;    -   c. minimize excision of mRNA regions by recognition by the        post-transcriptional machinery of mammalian host cell;    -   d. avoidance of single-stranded RNA folding due to resultant        mRNA sequence complementarity.

In the inventive method, the early (i.e., first region) of the geneutilizes codons most highly utilized by the mammalian host cell. Thisconsideration, therefore, improves the efficiency of gene expression byminimizing the likelihood of early termination of the ribosome. Althoughthe extent of the sequence that is left unaltered varies from gene togene, the region is typically up to 100 bp.

An important consideration is the fidelity of the tertiary structure andfolding of the protein produced in eukaryotic cells to the native,bacterially expressed protein. It is recognized that importantimmunogenic epitopes are likely to exist beyond the linear or evensecondary peptide structure. Rather, proper protein folding can bringamino acids or even peptide sequences, that are normally considerablydownstream of each other, into juxtaposition, creating importantimmunogenic conformational epitopes.

To improve the likelihood of producing these epitopes in the recombinantprotein, a search of the native PA sequence is undertake in order toascertain and identify regions containing relatively heavyconcentrations of rare codons. These regions may represent domains withspecific folding motifs within the normal, native PA protein. Therefore,retention of these regions in the modified sequence is incumbent uponsubstituting the rare bacterial codons with complimentary rare codonsfrom the human bias table. This would permit ribosome progression toslow, where it normally would in the bacteria, and permit normal foldingto occur.

In order to ensure against unwarranted deletions of mRNA the bacterialsequence is analyzed with the aim of identifying ribosomal splice sites.Alteration of these regions, therefore, will ensure thatpost-transcriptional machinery of mammalian cells does not inadvertentlydelete sections of the mRNA

Finally, an analysis of the bacterial sequence is undertaken to identifyany regions of complementarity. These regions are of importance sincethese regions could potentially result in single stranded RNA foldingback on itself, resulting in unwanted host cell operations, such asribosomal stalling or premature termination of translation.

Collectively, the inventive method avoids “over optimization” of thebacterial gene sequence. Instead, the method provides a more deliberateprocedure leading to an expressed protein with greater antigenicsimilarity to native, bacterially expressed protein.

Example Design of Humanized Bacillus anthracis Protective Antigen (PA)

In order to illustrate the inventive method, the human optimization ofPA (HoPA) was undertaken. As discussed above, the factors that wereconsidered in the development of the humanized gene sequence. Thefeatures of HoPA include:

-   -   a. highly used codons for the first 50 codons of the sequence,        thereby effectively engaging the ribosome and reducing premature        termination;    -   b. using rare codons from the human codon bias table in the same        positions where the wildtype PA gene sequence used rare codons        from the Bacillus anthracis codon usage table thus insuring that        where the bacterial ribosome paused during protein synthesis in        the bacteria, the mammalian ribosome did as well;    -   c. ensuring that regions, where there are many rare codons close        together, are maintained but the actual number of rare codons        reduced in order to minimize the likelihood of ribosomal        progression slowing or stalling;    -   d. a search for cryptic ribosomal splice sites was undertaken to        ensure that the post-transcriptional machinery of the mammalian        cells did not delete sections of the mRNA;    -   e. secondary structure determinations to ensure that the        resulting mRNA did not have long regions of complementarity that        would result in a single stranded RNA that was folded back onto        itself, into a secondary structure that could not be resolved by        the ribosome also leading to premature termination of the        translation process.

Human optimization was performed on the non-proprietary wild-type PAgene (GenBank Accession no. AAA22637.1). In designing the new sequence,the factors, above, were considered and incorporated into the new humanoptimized sequence (HoPA). The new HoPA gene sequence is illustrated inFIG. 1 (A-C) adjacent to the native sequence. The optimized sequence isalso listed in SEQ ID No. 1.

Unlike in the native sequence, the new nucleotide sequence lacks many ofthe rare codons and motifs that hinder expression in eukaryotes whileusing human rare codons to emulate the overall spacing of rare codons.An important consideration is to avoid over optimization of the genesequence. Over optimization may result in the most common eukaryoticcodons depleting the available reservoir of normally abundant tRNAs.This process may artificially accelerate the processivity of theribosome, increasing the chance that the nascent protein will not foldinto the proper secondary structure.

The newly synthesized PA gene also included Sap 1 restriction sites atthe N- and C-terminal ends to allow effective cloning into themulti-cloning site of the pDNAVACCultra2™ (Nature Technology, Lincoln,Nebr.) construct. At the same time, the amino terminal Bacillus leaderpeptide was eliminated since cloning into pDNAVACCultra2™ places thehuman TPA leader peptide upstream and in-frame of the PA sequence. Thismodification effectively increases extracellular trafficking of therecombinant PA (rPA) protein by eukaryotic cells. Ultimately, due tocodon redundancy, when both genes are translated they result in the samewild-type amino acid sequence, as illustrated in SEQ ID No. 1.

In order to evaluate the expression of HoPA in a eukaryotic cell lineChinese hamster ovary (CHO) cells strain K1 was transfected withpDNAVACCultra2™-HoPA and pDNAVACCultra2™-HoPA carrying the greenfluorescent protein (GFP). Efficient transfection and thetranscription/translation of HoPA were verified by the expression of GFPfrom the modified HoPA-GFP vector. Western blot analysis of supernatantsfrom the transfected CH0-K1 cells after 20 hr demonstrated the presenceof PA. This study confirmed that HoPA with it's codon optimizations,could be expressed from a eukaryotic cell line using the host cellmachinery.

Referring to FIG. 2, analysis of the mouse antibody response followingimmunizations with HoPA cloned into NTC's pDNAVACCultra2™ DNA vaccineexpression vector demonstrated that the animals had mounted PA specificIgG responses. The efficacy of the human TPA sequence at the beginningof HoPA, to direct the synthesized PA protein out of the cell fordetection and processing by circulating immune cells, was evaluated.

In the first study 50 μg pDNAVACCultra2™-HoPA or pDNAVACCultra2™-null(no insert) were mixed with a lipid adjuvant prior to beingintramuscularly injected into mice (n=8) on three separate occasions 28days apart. Serum anti-PA IgG titers were tracked for 56 days anddemonstrated that following the second homologous boost on day 28 therPA specific IgG titer increased from baseline to 33.8 μg/ml (FIG. 2).

In another animal study the efficacy following multiple administrationswas evaluated (FIG. 3). In this study efficacy was evaluated afterinjecting various doses (100, 75, 50, 25, and 12.5 μg) ofpDNAVACCultra2™-HoPA once, twice, and three times. Serum IgG titers weretracked as before. Dose and frequency related responses were observed,with the highest antigen specific titers (FIG. 3) achieved with thehighest dose of DNA injected three times. The same doses of vaccinegiven twice (days 28 and 42) (FIG. 4) or once did not generate as robustan IgG response as the triple vaccination schedule or rPA.

Two weeks after the last vaccination the mice were challengedintraperitoneally with 40 LD₅₀s of B. anthracis Sterne strain spores(4.03×10⁵ CFU/mouse). Survival was tracked over the course of 14 days(FIG. 5). Survival was significantly improved (p<0.05) with 100 μm ofpDNAVACCultra2™-HoPA (designated as 7162-HoPA in the figure below) whenadministered three times. Doses less than 100 μg or immunizationschedules that lacked a second booster are not efficacious. Incomparison, 10 μg of rPA protected 100% of the challenged mice. Theseresults speak to the efficacy of HoPA when cloned into the pDNAVACCultraconstruct. In FIG. 5, the negative control is the use of the adjuvantDioleoyl phosphatidylethanolamine-dimethyl dioctadecylammonium bromide(DDAB-DOPE).

FIG. 6 illustrates the immunoglobulin concentration followingimmunization with humanized PA. As seen in panel A, a significanthumoral response is evident following either recombinant PA protein orHoPA. However, significantly less antibody response is seen followingimmunization with the DNA expressed HoPA than following immunizationwith rPA protein. The clear dichotomy of immunoglobulin induction seenin FIG. 5 between HoPA (expressed from an administered DNA vector) andrPA protein administration induced between is likely dependent on theexpression efficiency of the DNA expression system.

Additionally, antibody induced by HoPA was capable of efficientlyneutralizing lethal toxin, as evidenced by toxin neutralization activity(TNA) assay. These results are summarized in Table 1. The assay wasconducted as described by Quinn, C. P., et al., J. Infect. Dis. 190:1228-1236 (2004). Prior to testing, recombinant PA (rPA) and recombinantLF (rLF) were titrated for toxin potency with J774A.1 cells. Theconcentrations of rPA and rLF that resulted in more than 99% cell lysisat a fixed cell density of 2×10⁴ cells/well were 45.1 and 36.1 ng/ml,respectively. Serum samples were serially diluted starting at 1:50 outto 1:102400 and were assayed in quadruplicate. The resulting serumneutralization curve (antibody dilution factor versus optical densitywas analyzed with a four-parameter logistic log fit curve. The primaryendpoint calculated from this 4-PL curve is the 50% effective antibodydilution (ED₅₀) that protects 50% of the eukaryotic cells in the assay.This value is reported as the reciprocal of the antibody dilutioncorresponding to the inflection point (“c” parameter) of thefour-parameter logistic log fit of the serum neutralization curve. AnED₅₀ greater than 200 is correlated with survival (Pitt, M. L., et al.,Vaccine 19: 4768-4773 (2001). Vaccination with pDNAVACCulture2-HoPAthree times elicited antibody responses by day 69 (FIG. 6A) withsufficiently high toxin neutralization capacities greater than 200 ED₅₀.

TABLE 1 Mouse ED₅₀ 1.3 587.8 1.4 409.4 1.7 1144.7 1.8 272.2

A further demonstration of the effectiveness of the humoral responseinduced by HoPA is illustrated in FIG. 6B. In FIG. 6B, mice wereimmunized with either rPA protein or plasmid-HoPA expression plasmid(pDNAVACCuItra™). In FIG. 6B, the survival of mice following anthraxchallenge, that had been immunized with HoPA, was equivalent to thatobserved for the recombinant protein, despite the much higher levels ofimmunoglobulin induced. The improved efficacy of the immunoglobulin overthat induced from rPA protein likely represents the greater similarityof HoPA to native PA structure. Additionally, HoPA may induce higheraffinity anti-PA antibody or higher concentrations of antibody specificto regions on the native PA molecule that are more relevant to immuneprotection. One explanation is that HoPA may induce helper T-cellresponse to induce IL4. This may suppress IgG2a and IgG2b responses andincrease IgG1.

Although the example shown expressed the novel PA in the pDNAVACCultra™construct, expression of the optimized PA construct can be inserted andexpressed by other suitable expression systems. This could include otherDNA and viral expression systems.

1. An immunogenic composition comprising a recombinant Bacillusanthracis protective antigen gene, wherein said composition is encodedby a nucleic acid sequence wherein regions of said sequencecorresponding to regions of B. anthracis containing rare bacterialcodons are replaced with rare mammalian codons and where the codons inthe first 2 to 5% of the sequence are highly utilized human codons. 2.The immunogenic composition of claim 1, wherein the mammalian codons arehuman.
 3. The immunogenic composition of claim 1, wherein the first 50codons are highly utilized human codons.
 4. The immunogenic compositionof claim 1, wherein said amino acid sequence is SEQ ID No. 1 and isencoded by nucleic acid sequence of SEQ ID No.
 2. 5. The immunogeniccomposition of claim 1, wherein said DNA is expressed from a DNA orviral expression system.
 6. The composition of claim 1, wherein proteinproduct encoded from said recombinant protective antigen gene is inducedvia the TPA signal.
 7. A method of modifying a recombinant bacterialgene for expression in a mammalian host comprising: a. replacing highlyutilized host codons in the first 2 to 5 percent of the total bacterialgene sequence; b. replacing regions of the sequence where there arestretches of three or more rare bacterial codons with rare host codons;c. evaluation said modified bacterial gene sequence to identify regionsof complementarity that could result in RNA folding back on itself; d.removing said regions of complementarity by altering the codons in theseregions but retaining the same amino acid sequence of the expressedprotein; and e. evaluating for cryptic ribosomal splice sites in orderto avoid regions of said gene being deleted by the host cell; f.altering regions containing said splice sites to remove recognitionsequences but retaining the original amino acid sequence.
 8. A method ofinducing an immune response against Bacillus anthracis comprising: a.administering to the immunogenic composition of claim 1; b.Administering a second, boosting dose of said immunogenic composition.9. The method of claim 8, wherein said immunogenic composition isexpressed from a DNA or viral expression vector.